DEV: Use the right number of cores for bundle install in tests workflow (#26389)

Why this change?

We run on different runners depending on the scenario. We should use the
right number of parallel jobs for bundle install based on the number of
CPU cores the runner has.

.github/workflows/tests.yml

@@ -96,7 +96,7 @@ jobs:
           bundle config --local path vendor/bundle
           bundle config --local deployment true
           bundle config --local without development
-          bundle install --jobs 4
+          bundle install --jobs $(($(nproc) - 1))
           bundle clean
 
       - name: Get yarn cache directory